A typical on-board evaporative emission control system comprises a vapor collection canister that collects fuel vapor emitted from a tank containing volatile liquid fuel for the engine and a CPS valve for periodically purging collected vapor to an intake manifold of the engine. In a known evaporative system control system, the CPS valve comprises a solenoid that is under the control of a purge control signal generated by a microprocessor-based engine management system. A typical purge control signal is a duty-cycle modulated pulse waveform having a relatively low operating frequency, for example in the 5 Hz to 20 Hz range. The modulation may range from 0% to 100%. This means that for each cycle of the operating frequency, the solenoid is energized for a certain percentage of the time period of the cycle. As this percentage increases, the time for which the solenoid is energized also increases, and therefore so does the purge flow through the valve. Conversely, the purge flow decreases as the percentage decreases.
Changes in intake manifold vacuum that occur during normal operation of a vehicle may also act directly on a CPS valve in a way that upsets the intended control strategy unless provisions, such as a vacuum regulator valve for example, are included to take their influence into account. When the CPS valve is closed, manifold vacuum at the valve outlet is applied to the portion of the valve element that is closing the opening bounded by the valve seat. Changing manifold vacuum affects certain operational characteristics of such a valve, potentially causing unpredictable flow characteristics.
The particular construction of a solenoid-actuated valve, and certain external influences thereon, may impair certain operational characteristics, such as the start-to-flow point and the incremental low-flow characteristic.
From commonly assigned U.S. Pat. No. 5,413,082, inter alia, it is known to incorporate a sonic nozzle function in a CPS valve to reduce the extent to which changing manifold vacuum influences flow through the valve during canister purging. From U.S. Pat. No. 5,373,822, it is known to provide pressure- or force-balancing of the armature/valve element.
From other patents, such as commonly assigned U.S. Pat. No. 4,901,974 issued Feb. 20, 1990, it is known to incorporate noise-attenuating bumpers to absorb impact forces created by abutment of the armature with stops as the armature reciprocates.